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Australian counterterrorism police said they arrested a 24-year-old man in Sydney yesterday on suspicion of planning a terrorist attack, the latest in a string of arrests connected with radical Islamist-inspired violence.

"A Bankstown man was arrested today by the Joint Counter Terrorism Team (JCTT) Sydney as part of the ongoing Operation Appleby investigation into the alleged planning of a terrorist attack in Australia," the Australian Federal Police said in a statement.

"It will be alleged in court the man was one of a number of people - who have been previously charged regarding this matter - involved in formulating documents connected with preparations to facilitate, assist or engage a person to undertake a terrorist act."

Last week, police carried out raids across Melbourne in connection with five men accused of planning to travel to Syria to join Isis (Islamic State) via a journey that would start with a motor boat trip from Australia to Indonesia.

The men, aged between 21 and 31, were charged with preparing to enter a foreign country "for the purpose of engaging in hostile activities", an offence that carries a maximum penalty of life imprisonment.

The five, who were not identified, were arrested after towing a 7m motor boat almost 3000km from Melbourne to Cairns.

In an unrelated incident on the same day, police in Sydney arrested an 18-year-old man for allegedly planning to obtain a gun to commit a terrorist act targeting public buildings or landmarks in the city.

Australia, a staunch US ally, has been on heightened alert for attacks by home-grown radicals since 2014 and authorities say they have thwarted a number of plots.

There have been several lone-wolf assaults, including a 2014 cafe siege in Sydney that left two hostages and the gunman dead.

About 100 people have left Australia for Syria to fight alongside organisations such as Isis, Australia's Immigration Minister said last month.